1. Field of the invention
The present invention relates to a polarization conversion device having a half-wave (λ/2) plate for separating two beams of linear polarized light orthogonal in the direction of polarization into coincidence in the direction of polarization with each other and bringing the separated two beams into coincidence with each other, a method of manufacturing the polarization conversion device, and an illumination source device using the polarization conversion device.
2. Description of Related Art
It is popular to use a polarization conversion device for coordinating illumination light beams from a light source in optical systems such as an LCD projection display system as described in, for example, Unexamined Japanese Patent Publication (TokKai) No. 2001-235624, issued Aug. 31, 2001. Reference is made to FIGS. 1(A) and 1(B) showing such a polarization conversion device for the purpose of providing a brief background that will enhance an understanding of the present invention. A polarization conversion device 10 shown in FIG. 1(A) functions as both means for separating p- and s-polarized light, taking the form of a polarizing beam splitter, and means for redirecting one of the polarized beams, taking the form of a right angle prism reflector. Specifically, the polarization conversion device 10 comprises a number of prismatic beam splitting elements 16 having a parallelogram cross section arranged in a straight row. Each of the prismatic beam splitting elements 16 has opposite end surfaces one of which forms a part of an incidence surface 12 of the polarizationconversion device 10 and the other of which forms a part of an exit surface 14 of the polarization conversion device 10, and opposite side surfaces one of which is provided with a polarizing beam splitter (PBS) layer 24 coated thereon and the other of which works as a right angle internal reflection surface. The polarizing beam splitter (PBS) layer 24 partly transmits and partly reflects polarized light to separate two linear polarized light, namely p- and s-polarized light, which are orthogonal in the direction of polarization. Further, the polarization conversion device 10 is provided with half-wave (λ/2) plate strips 18 attached to exit surfaces of every other prismatic beam splitting elements 16. Each of the half-wave (λ/2) plate strips 18 rotates the polarized light incident thereupon 90 degrees.
As shown in FIG. 1(B), linearly polarized light is collimated by some means and enters the polarization conversion device 10. The polarizing beam splitter (PBS) layer 24 of each of the prismatic beam splitting elements 16 transmits the p-polarized light and reflects the s-polarized light. The transmitted p-polarized light travels in another prismatic beam splitting 16 adjacent thereto and exits the adjacent prismatic beam splitting element 16, where it is then rotated 90° in the angle of polarization by the half-wave (λ/2) plate strip 18, and hence converted to s-polarized light On the other hand, the reflected s-polarized light is turned 90° by the internal reflection surface and exits the polarization conversion device 10. The adjacent and spatially separated collimated light beams exiting from the polarization conversion device 10 have been coincident in the direction of polarization (in this case, into an s-polarized state).
A drawback, notably in the case of alternately arranged half-wave (λ/2) plate strips 18, is the fact that light in one of p- and s-polarized states (in this case, p-polarized light) entering partial areas of the incidence surface 12 of the polarization conversion device 10, corresponding to projections of the half-wave (λ/2) plate strips 18 on the incidence surface 12 (i.e. end surfaces of the every other prismatic beam splitting elements 16 which are not provided with the half-wave (λ/2) plate strip 18), can not be converted in the direction of polarization (in this case, into an s-polarized state). Accordingly, a total available area of the incidence surface 12 of the polarization conversion device 10 (working effectively on conversion of the direction of polarization is substantially half as much as the whole incidence surface 12.
There have been proposed various approaches for increasing the available incidence area of the incidence surface of such a conventional polarization conversion device as described above. For example, in the case where a number of small light source or elements such as LEDs are included in a light source device, a plurality of light emitting elements are longitudinally arranged in a column with their optical axes spatially laid along each available incidence area of the surface of incidence 12 of the polarization conversion device 10 so as thereby to project illumination on a locally restricted area, i.e. the available incidence area, of the surface of incidence 12. In the case where a single light source of large size is employed, a fly-eye lens array comprising a number of small lenses is put between the light source and each available incidence area of the surface of incidence 12 of the polarization conversion device 10 so as to provide a number of secondary light sources arranged in columns with their optical axes spatially laid along the available incidence area of the surface of incidence 12, respectively. This single source unit can project illumination on the respective available incidence area of the surface of incidence 12 efficiently. For a more complete description of this proposed solution, see Unexamined Japanese Patent Publication (Tokkai) No. 11-218724, issued Aug. 10, 1999.
Incidentally, a preferable approach for forming an efficient and compact configuration of small size light source elements or small lenses forming the fly-eye lens array is to arrange them in a checked pattern wherein adjacent rows of the light source elements or the lenses are shifted half a pitch from each other. In particular, the small size light source elements are often arranged in a checked pattern in light of the necessity of arranging the light source elements as many much as possible in a specified area. However, because the conventional polarization conversion device has available incidence areas of the incidence surface each of which is long in a lengthwise direction, it id hard to lay optical axes of the light source elements arranged in a checked patter on the available incidence areas.